The present invention relates generally to the art of amplifier circuits and more particularly to a fully differential current feedback amplifier.
In broadband and wireless communications systems, information is transferred between sources and destinations through various communications media. For example, in wireless systems, analog signals carrying information are transmitted to and received from the ambient via an antenna. When such a communications signal is received, it is initially processed in an analog signal chain between the receiving antenna and a digital processing system. An analog to digital (A/D) converter digitizes the processed analog signal, which is then subjected to signal processing to obtain the received signal information. The analog signal chain operates to condition, filter, and amplify the received analog signal, and to provide frequency conversion as necessary in order to reduce noise and to provide a proper input signal to the A/D converter. Recently, the frequencies of signals within such circuits are increasing in order to provide improved speed, conforming to communication protocols or standards, etc. Consequently, analog signal chains and particularly amplifiers thereof in modern communications systems are required to operate at ever higher frequencies with larger dynamic range, decreased distortion, and lower power supply voltages.
In a typical communications system, an antenna signal is provided to a low noise amplifier (LNA), and from there into a mixer that takes the received signal from the transmission frequency down to an intermediate frequency. More than one such frequency down-conversion may take place, for example, via subsequent mixer stages. Various filters, such as band-pass filter stages, are typically provided between the antenna, amplifiers, and mixers, in order to reduce or remove noise and signals that are outside the frequency band of interest. Once the resulting analog signal has been converted to a manageable frequency, it is amplified once again and provided to an A/D converter. In order to perform a proper A/D conversion so as to extract all the information of interest from the signal, it is desirable to provide the A/D with a very large signal. Thus, one or more amplifiers are found in the typical analog signal chain to amplify the relatively weak signal at the antenna and/or at intermediate stages, wherein the A/D converter receives a signal which is slow enough to digitize, and also amplified so that the A/D can digitize all the fine signal details.
Initially, a relatively weak single-ended signal is obtained from the receiving antenna. Differential signals have many advantages over single-ended signals in such systems, including increased dynamic range for a given supply voltage, suppression of even order harmonic distortion, and enhanced common mode noise rejection. Thus, at some point in the analog signal chain, the single-ended signal is converted into a differential signal. The earlier that the conversion from single-ended to differential takes place in the signal chain, the earlier the benefits of differential signals are realized in the communications system. A portion of a conventional heterodyne type receiver system 10 is illustrated in prior art FIG. 1, where a heterodyne receiver translates the desired radio frequency (RF) signal to one or more intermediate frequencies before demodulation. The exemplary receiver system 10 includes several active and passive function blocks and each contributes to the system""s overall signal gain and noise figure (NF). The receiver system 10 of FIG. 1 includes an antenna 12, a duplexer 14, an amplifier 16, one or more filters 18a and 18b, a mixer 20 driven by a local oscillator (LO) 21, a second amplifier 24, and an A/D converter 26.
The antenna 12 provides an interface between free space and the receiver input. The duplexer 14 interfaces with the antenna 12 and allows simultaneous transmitter and receiver operations with a single antenna, wherein the duplexer 14 operates to isolate the receiver system 10 and a transmitter 22 from each other while providing a generally low loss connection to the antenna 12 for both systems. The system 10 also includes a first amplifier 16, typically a low-noise amplifier (LNA), which increases the amplitude of the signal received from the antenna 12 allowing further processing by the receiver 10. An ideal amplifier increases the amplitude of the received signal without adding distortion or noise. Real world amplifiers, however, add noise and distortion to the received signal, and attempts are made to minimize signal degradations. The LNA 16 is the first amplifier after the antenna 12 in the system 10 and contributes most significantly to the system noise figure, consequently the amplifier 16 is typically designed to minimize noise, and hence the name LNA.
Band-pass type filters 18a and 18b form one or more networks which allow a range of RF frequencies to pass therethrough, while blocking RF signals outside of their designed passband. The filter 18a located before the LNA 16 is called a preselect filter and the post-amplifier filter 18b is often called an image-reject filter. The preselect filter 18a prevents signals far outside of the desired passband from saturating the front end of the amplifier 16 so as to reduce intermodulation distortion products related to those signals at far away frequencies, while the image-reject filter 18b rejects spurious response type signals. The mixer 20 translates the received, filtered, and amplified RF signal to both a higher and lower intermediate frequency (IF) value. One of the intermediate frequencies is passed while the other is rejected (e.g., called either up-conversion or down-conversion, respectively), using translation with a signal from the local oscillator 21, which mixes with the RF signal. Thereafter, a second amplifier 24 further amplifies the analog signal from the mixer 20 and provides an input to the A/D converter 26.
Many conventional mixers are designed to receive a differential input because differential signals help in decoupling the system 10 from noise in the integrated circuit substrate, thereby lowering the system NF, and aid in facilitating high device integration. Where the mixer 20 is designed to receive a differential signal input and the antenna 12 generates a single-ended received signal, the system 10 must transform the single-ended signal into a differential signal somewhere between the antenna 12 and the mixer 20. Conventional solutions which perform a transformation from a single-ended signal to differential signals before the LNA 16 have been found undesirable because prior to amplification the received signal is weak and the transformation results in too much loss, thereby degrading the integrity of the received signal. Similarly, conventional post-LNA transformation solutions have been found to be undesirable because of linearity issues. Alternatively, the single-ended to differential conversion can be performed in either the first LNA 16 or in the second amplifier 24 supplying a differential input to the A/D converter 26.
Conversions or translations of single-ended signals to differential signals have sometimes been accomplished using differential voltage feedback amplifiers or current feedback amplifiers, each having particular shortcomings. In the first case, the amplifiers 16 and/or 26 in FIG. 1 may be such a voltage feedback type amplifier. However, voltage feedback amplifiers suffer from various drawbacks in high-speed applications. For instance, the bandwidth in differential voltage feedback amplifiers is dependent upon gain, thus limiting design flexibility for gain adjustment where a large bandwidth is contemplated. For example, with a voltage feedback amplifier, as the amplifier closed loop gain is increased, the speed of the signals it is able to support decreases. Thus, conventional voltage feedback amplifiers are typically slower (e.g., with respect to the bandwidth that they""re capable of achieving, and the slew rates that they""re capable of supporting) than are current feedback type amplifiers.
Alternatively, as illustrated in FIG. 2, two current feedback type amplifiers 30 and 32 have sometimes been employed with appropriate feedback resistors 34-37 to provide a differential output VOUT based on a single-ended input VIN. Current feedback amps are usually able to handle signals that are coming in faster and swinging higher, compared with the voltage feedback amplifiers. The current feedback amplifiers 30 and 32 do not suffer from the gain-bandwidth dependencies experienced with voltage feedback amplifiers. However, this current feedback amplifier approach requires a transformer 38 to couple the single-ended signal VIN to the individual current feedback amplifiers 30 and 32, resulting in increased cost, physical space, and other problems associated with transformer 38. For instance, the transformer 38 itself may degrade system performance with respect to noise and/or signal loss, and may have inherent bandwidth limitations.
Thus, designers of modern communications system analog signal chains have heretofore been forced into a design tradeoff between the relative strengths and weaknesses of either voltage feedback amplifiers or current feedback amplifiers. As a result, there is a need for improved amplifiers for differential or single-ended inputs, which reduces or mitigates some or all the above-mentioned difficulties previously experienced with voltage feedback amplifiers or current feedback amplifiers in high-speed communications system applications such as digital subscriber line (DSL) and wireless communications systems, such as mobile battery operated systems and high performance base stations.
The following presents a simplified summary in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor to delineate the scope of the invention. Rather, the primary purpose of this summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention involves differential current feedback amplifiers, which provide two low impedance inputs, and two differential outputs which are about 180 electrical degrees out of phase with respect to one another, wherein feedback loops use currents rather than voltages as feedback error signals. This is in contrast to voltage feedback amplifiers having two high impedance inputs, and conventional current feedback amplifiers having one high impedance input (e.g., typically the non-inverting input) and one low impedance input (e.g., the inverting input). Thus, the invention contemplates fully differential current feedback amplifiers, which provide a differential output based on differential input signals or a single-ended input, without the need for an external transformer. The differential current feedback amplifier further provides speed and gain-bandwidth independence advantages associated with prior current feedback amplifiers. Thus, the invention finds application in high-speed communications and other systems, allowing system designers significantly improved design flexibility in selecting gains and bandwidths without sacrificing performance. In addition to communications systems, the differential current feedback amplifiers of the invention find application in other systems as well.
In one aspect of the invention, a differential current feedback amplifier is provided having low impedance inputs to receive first and second input signals, and first and second phase shifting systems or circuits providing first and second phase shifted input signals based on the second and first input signals, respectively. In one implementation, the phase inversion aspects of the invention may involve current steering amplifier circuitry having a constant current source and two identical collector connected transistors. The first input signal and the first phase shifted signal are combined or summed in order to provide a first intermediate signal and the second input signal and the second phase shifted signal are summed to provide a second intermediate signal. Each intermediate signal thus represents the combination (e.g., summation) of one input and a phase shifted version of the other input.
The intermediate signals are then buffered to provide differential outputs. The first differential output thus represents the first input signal and a phase shifted version of the second input signal. Likewise, the second differential output signal represents the second input signal and a phase shifted version of the first input signal. The differential current feedback amplifier may thus be connected so as to amplify a differential input signal, or to convert a single-ended input signal into an amplified differential output signal. In either configuration, the gain is largely independent of bandwidth, thus providing advantages in high-speed system design.
To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.